Yes, that was my point - it is not a certainty (and so any advice beginning "since your project is open source..." might be based on a misconception.)

>...whether the employee worked on it during company time, whether the employee was asked by the employer to work on it and how closely the work aligns with the employer's business.

It actually depends on the terms of OP's contract and whether the relevant clauses (if any) are enforcible in the appropriate jurisdiction. The factors you mention may be a factor in whether this is open source (and we already know, from OP, that it does align with the employer's business), but the "but if..." argument of your second paragraph is unlikely to carry much weight, legally. This is why OP should speak to a lawyer if she is dissatisfied with the employer's proposal.